The invention also relates to the use of the catalyst in a process for isomerizing aromatic compound containing 8 carbon atoms per molecule.
Isomerisation of ethylbenzene to xylenes requires the presence of a group VIII metal. Optimised formulations based on mordenite and a group VIII metal have produced catalysts wherein side reactions remain non negligible. Examples are naphtene ring opening followed or otherwise by cracking, or dismutation and transalkylation of C.sub.8 aromatics which lead to the formation of unwanted aromatics. The development of more selective novel catalysts is thus of particular importance.
ZSM-5 is one zeolite which is used for isomerisation of C.sub.8 aromatic cuts, used alone or mixed with other zeolites such as mordenite. Such catalysts have been described in United States patents U.S. Pat. No. 4,467,129, U.S. Pat. No. 4,482,773 and European patent EP-B-0 138 617. Other catalysts are based on mordenite and have been described in U.S. Pat. No. 4,723,051, U.S. Pat. No. 4,665,258 and French patent FR-A-2 477 903.
The lack of selectivity of mordenite can be attenuated by optimising specific treatments and/or formulations as described, for example, in our French patent FR-A-2 691 914. Such techniques can reduce dismutation side reactions.
EU-1 zeolite with structure type EUO, which has already been described in the prior art, has a unidimensional microporous framework, with a pore diameter of 4.1.times.5.7 .ANG. (1 .ANG.=1 Angstrom=10.sup.-10 m) ("Atlas of Zeolite Structure Types", W. M. Meier and D. H. Olson, 4.sup.th edition, 1996). Further, N. A. Briscoe et al. stated in their article in the review Zeolites (1988, 8, 74) that such unidimensional channels have lateral pockets with a depth of 8.1 .ANG. and a diameter of 6.8.times.5.8 .ANG.. A method for synthesising EU-1 zeolite and its physico-chemical characteristics have been described in European patent EP-B 1-0 042 226. United States patent U.S. Pat. No. 4,640,829 relates to ZSM-50 zeolite which, according to the "Atlas of Zeolite Structure Types", W. M. Meier and D. H. Olson, 4.sup.th edition, 1996, has the same EUO structure type as EU-1 zeolite. That patent describes a method for synthesising ZSM-50 which is different from that described in EP-B1-0 042 226 for EU-1 zeolite. EP-A1 0 051 318 relates to TPZ-3 zeolite which, according to the "Atlas of Zeolite Structure Types", W. M. Meier and D. H. Olson, 4.sup.th edition, 1996, has the same EUO structure type as EU-1 zeolite, and its use as a catalyst containing zeolite as it is or formed. In that document, forming of the TPZ-3 zeolite is exemplified by preparing pellets, obtained by pelletizing a mechanical mixture of zeolite powders and a binder. The pellets contain the TPZ-3 zeolite, a binder and optionally at least one element selected from the group formed by iron, cobalt, nickel, copper, zinc, ruthenium, rhodium, palladium, rhenium, osmium, iridium and platinum, as the metal or metal oxide.